# Readme

This is a test to get a PIC32MZ2048ECG064 running without the MPLAB IDE. There
is an Assembly file and a Makefile. The generated HEX can be uploaded using the
MPLAB IPE. Generating C code is fairly similar, but uses `gcc` instead of `as`.

### Links

About the chip: https://www.microchip.com/wwwproducts/en/PIC32MZ2048ECG064  
About the architecture: https://www.cs.cornell.edu/courses/cs3410/2008fa/MIPS_Vol1.pdf  
About the instruction set: https://www.cs.cornell.edu/courses/cs3410/2008fa/MIPS_Vol2.pdf

## Notes to self

### Instruction set

This chip has a somewhat larger instruction set than MIPS32; it is a MIPS32
microAptiv. For this extension, see §50 of the PIC32 reference manual (to be
found on the chip information page linked above). It is not clear to me how
widespread this instructions set is (i.e. whether it would make sense to rely
on it being available for code generation or so).

### Branch delay slots

Watch out for the branch/jump instructions. The following instruction is always
executed, regardless of whether the branch is taken or not. This instruction is
said to be in the 'branch delay slot'. For example, this does not loop
endlessly even though the body of `main_loop_delay` seems empty:

```
	lui	$1,0xf
main_loop_delay:
	bne	$1,$0,main_loop_delay
	addiu	$1,-1
```

Also note that when the file ends with a branch/jump, a `nop` *must* follow to
avoid unpredictable behaviour (the following instruction could be anything
otherwise).

### `lw` and `lui`

One needs `lui` to load an address, e.g. to clear `TRISB`:

```
	lui	$2,%hi(TRISB)
	sw	$0,%lo(TRISB)($2)
``` 

Using `lw` instead of `lui` is an easy mistake, assembles fine, but you usually
end up reading out of memory leading to a reset.